Something is leaving droppings and some sort of wood pulp stuck to the walls and shelves in my closet. I think whatever it is might be coming out of the ceiling. There's a foot or so of empty space between my apartment and the one above me where they run the air ducts, wires, etc. I taped a penny to the wall in the first picture for a size reference.
I have an access panel to the shower in my closet. The panel wasn't properly attached, and used to fall off. I thought something was coming from there originally, but I glued the panel in place and cleaned everything up several months ago, and then discovered this again today.
Since the location is probably relevant, I'm in Houston, TX.